When it is desired to shift a beam spot formed on a disc of an optical disc apparatus, such as a so-called compact disc player, from the current track to another optical track, an optical pickup head itself is moved, if the amount of shifting is large, by way of performing a so-called track jump control, or the optical pickup head is not driven, and only an objective lens in the optical pickup head is turned to shift the beam spot to the desired track, if the amount of shift is small, by way of performing a so-called seek control.
Amount the conventional seek control apparatus, there is known such an apparatus in which the frequency signal indicating detection of a tracking error is subjected to a frequency-voltage conversion, a feedback is applied in dependence upon the resulting voltage, the number of pulses of the frequency signal indicating the detection of the tracking error is counted and the seek operation is terminated when a predetermined track count is reached. However, this seek control apparatus suffers from disadvantages that the frequency to voltage conversion cannot be achieved with high accuracy, the direction in which the objective lens is turned to shift the beam spot cannot be identified, and the counting error is apt to be produced because feedback cannot be applied to an up/down counter (43) directly. For this reason, the seek control apparatus shown in FIG. 4 is employed.
The seek control apparatus shown in FIG. 4 is made up of an up/down counter (43), a D/A converter 44, a phase compensation circuit (45), a driving circuit 46 and a biaxial coil (47). For detecting the current position of the beam spot, the seek control apparatus makes use of a tracking error signal indicating the detection of a tracking error, as shown at (a) in FIG. 5, and a pull-in signal which, conversely to the tracking error signal, reaches the maximum level at the tracking center position, as shown at (c) in FIG. 5.
The tracking error signal and the pull-in signal are waveformed, as shown at (b) and (a) in FIG. 5. From these two waveformed signals, an up/down changeover signal is produced on the basis of the phase difference of these two signals. This up/down changeover signal is supplied to an up/down signal input terminal (43b) of the up/down counter (43) by means of an input terminal (42).
To a clock input terminal (43a) of the up/down counter (43), the aforementioned tracking error signal, for example, is supplied. The up/down counter (43) is responsive to the up/down changeover signal to count up or count down to supply the count output to a D/A (digital/analog) converter (44).
The counter (43), loaded with the value of the target track to be sought, is adapted to continue its counting operation until this value is reached.
The aforementioned position information signal is converted from the digital signal into the analog signal at D/A converter (44) and thence supplied to a phase compensation circuit (45). The position information signal is phase-compensated in this phase compensation circuit (45) and thence supplied to a driving circuit (46). This driving circuit 46 actuates the objective lens into a turning or swinging motion by the driving current supplied to a biaxial coil (47). The objective lens driving operation is terminated when the up/down counter (43) has counted up to the target value of the track being sought.
The above described conventional seek control apparatus is extremely difficult to control inasmuch as the travelling speed of the beam spot as well as settling properties since the time of travel until stabilization is governed by the step response of the circuitry inclusive of the phase compensation circuit (45), control difficulties are increased when the drive voltage of the step response is saturated.
In view of the above mentioned inconveniences, it is an object of the present invention to provide a seek control apparatus in which the beam spot movement may be achieved at a higher speed and with satisfactory settling properties.